The heat exchangers of radiators, air conditioners, intercoolers and oil coolers in automobiles are assembled by brazing together working fluid conduit component materials consisting of Al—Cu alloys, Al—Mn alloys, Al—Mn—Cu alloys and the like with fins consisting of Al—Mn alloys and the like. The fin materials need to have a sacrificial anode effect in order to protect against corrosion of the working fluid conduit component materials, and must have sag resistance and erosion resistance to prevent deformation or permeation of braze due to the high temperatures attained during brazing.
The reason that Al—Mn aluminum alloys such as JIS 3003 and JIS 3203 are used as fin materials is that Mn functions effectively to prevent deformation and corrosion during brazing. In order to provide an Al—Mn alloy fin material with a sacrificial anode effect, there is a method of adding Zn, Sn or In to the alloy to make it electrochemically anodic (Patent Document 1 (Japanese Patent Application, First Publication No. S62-120455)), and in order to further raise the high-temperature sag resistance, there is a method of adding Cr, Ti or Zr in the Al—Mn alloy (Patent Document 2 (Japanese Patent Application, First Publication No. S50-118919)).
On the other hand, in recent years, there has been an increased demand for lightening and cost reduction of heat exchangers, and it is becoming necessary to make the heat exchanger component materials such as working fluid conduit component materials and fin materials even thinner. However, making the fins thinner reduces the thermal conductivity due to small cross section and thus decreases the heat exchange efficiency, and can cause problems in terms of strength and durability of the heat exchangers when actually subjected to use, so that better thermal conductivity, strength after brazing, sag resistance, erosion resistance and self-corrosion resistance are desired.
With conventional Al—Mn alloys, the Mn forms a solid solution due to the application of heat during brazing, thus reducing thermal conductivity. As a fin material capable of overcoming this difficulty, an aluminum alloy with the Mn content restricted to 0.8 wt % or less and containing 0.02-0.2 wt % of Zr and 0.1-0.8 wt % of Si has been proposed (Patent Document 3 (Japanese Patent Publication, Second Publication No. S63-23260)). While this alloy has improved thermal conductivity, it has the drawback of containing little Mn, so that the strength after brazing is insufficient, and it is susceptible to fin damage and deformation during use as a heat exchanger, and the electrical potential is not anodic enough so that the sacrificial anode effect is small.
On the other hand, by increasing the cooling speed when casting a slab by pouring an aluminum alloy melt, the size of intermetallic compounds crystallizing in the slab stage can be made small with a maximum value of 5 μm or less even if the Si and Mn content is made 0.05-1.5 mass %, and a process of rolling from such a slab has been proposed to improve the fatigue properties of the fin material (Patent Document 4 (Japanese Patent Application, First Publication No. 2001-226730)). However, the purpose of this invention is to improve the fatigue lifetime, and while there is a description to the effect that the cast slab can be made thinner as means for increasing the cooling speed when casting the slab, there is no specific disclosure such as of continuous casting of thin slabs by twin belt casting machines under actual operation.    Patent Document 1: Japanese Patent Application, First Publication No. S62-120455    Patent Document 2: Japanese Patent Application, First Publication No. S50-118919    Patent Document 3: Japanese Patent Publication, Second Publication No. S63-23260    Patent Document 4: Japanese Patent Application, First Publication No. 2001-226730